The invention relates to a hydraulically operated impact device comprising a frame and a percussion piston arranged to reciprocate in the frame due to the pressure fluid, the piston comprising at its upper end a cylindrical guide that has a smaller diameter than the greatest diameter of the percussion piston, the upper end of the guide comprising an impact pressure surface, the device further comprising an inlet duct and a return duct for supplying pressure fluid into and out of the impact device, a control pressure space situated near the upper end of the percussion piston, the guide of the percussion piston being arranged to enter said space substantially tightly at the end of the return motion of the piston, and a control valve for controlling the movement of the piston, the control valve closing in the return position the inlet duct for pressure fluid and opening the return duct, the control valve comprising a pressure surface that continuously communicates with the control pressure space, and a pressure surface that acts in the opposite direction and that is continuously connected to a pressure space situated further from said pressure space in the direction of impact of the percussion piston, the percussion piston closing the connection between said pressure surfaces of the control valve as it moves into the control pressure space and pushing the control valve from the return position to the impact position via the pressure fluid in the control pressure space, so that a connection is opened from the inlet duct for pressure fluid to said space, the pressure of the pressure fluid in the inlet duct acting on both the impact pressure surface of the percussion piston and said pressure surface of the control valve, causing the impact motion of the percussion piston.
Hydraulically operated impact devices, such as percussion hammers and other breaking apparatuses, are used for example to break relatively hard materials, including stone, concrete, asphalt, frozen soil, metal slag etc. For instance percussion hammers are usually installed as auxiliary equipment in excavators instead of buckets, but other base machines and carriers can also be used. Impact devices are usually operated by the hydraulics of the base machine. Correspondingly, hydraulically operated drilling machines comprising an impact apparatus are used to drill different rock materials.
An impact device comprises a hydraulically reciprocating percussion piston, which delivers successive blows via a tool at the object to be broken. Pressure fluid to and from the percussion piston is supplied by means of suitable ducts. The flow of the pressure fluid is guided to the operating space of the percussion piston according to the cycle of operation of the impact device for example by means of different spool valves and slides. However, the operation of known control means requires the supply of separate control pressure, which in turn requires the formation of complicated ducts and grooves that increase the number of leakages inside the device. Another problem of the present slides and other components operated by control pressure is the bypassing of the pressure fluid, in other words pressure fluid used for the control is released into a return duct of the pressure fluid circuit. This naturally causes unnecessary power consumption. Other prior art arrangements include different springs and other mechanical means for moving the control valve, but such arrangements are complicated, however, regarding both operation and manufacture, not to mention durability.
European Patent Specification 0,085,279 discloses a known arrangement for controlling the movements of the percussion piston. In this reference, the percussion piston is surrounded by a sleeve-like control valve, which controls the flow of the pressure fluid supplied to the percussion piston. In the apparatus according to the reference, when the upper section of the percussion piston forces its way into the sleeve as the piston rises, the upper surface of the sleeve is subjected to a pressure which makes the sleeve move downwards, whereupon a connection to a high-pressure duct is opened and high pressure starts acting on the upper end of the percussion piston, pressing it downwards to deliver an impact. The high pressure simultaneously acts on the upper surface of the sleeve, pressing it downwards all the way to its lower position. When the uppermost section of the percussion piston emerges from the sleeve, the same high pressure that pushes the percussion piston downwards starts acting on the surfaces at the lower part of the sleeve. The size of the pressure surface at the sleeve's lower end is greater than the size of the pressure surface at the upper end, which means that the sleeve starts moving upwards, closing again in its upper position the connection to the high-pressure duct. However, a problem of this arrangement is that the control valve strikes either the components bordering its area of movement or the bottom, which may damage the structure and cause unnecessary wearing in the long run.